The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Internal combustion engines combust an air/fuel (A/F) mixture within cylinders to drive pistons and generate drive torque. The combustion of the A/F mixture produces exhaust gas which may be expelled from the cylinders through exhaust valves and an exhaust manifold. The exhaust gas may include carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx). Exhaust treatment systems may be implemented to reduce CO, HC, and/or NOx in the exhaust gas. For example, an exhaust treatment system may include, but is not limited to, an oxidation catalyst (OC), a particulate matter filter (PMF), and a selective catalytic reduction (SCR) system. The OC oxidizes CO and HC to form carbon dioxide (CO2) and water (H2O). The PMF removes particulate matter from the exhaust gas. The SCR system reduces NOx in the exhaust gas.
The SCR system may use a reductant to reduce NOx emissions. For example, the reductant may be ammonia (NH3). In an SCR process, NOx reacts with the reductant to be absorbed onto an SCR catalyst. The SCR catalyst may also be referred to as an SCR material. The reductant may be introduced into the exhaust stream by injecting a reducing agent using a dosing system. For example only, the reducing agent may be pure anhydrous ammonia, aqueous ammonia, or urea. The injected reducing agent breaks down in the exhaust gas to form the reductant that is utilized to react with the NOx. The following exemplary chemical relationships may describe the NOx reduction:4NO+4NH3+O2→4N2+H2O2NO2+4NH3+O2→3N2+6H2OThe SCR process reduces NOx in the exhaust gas, forming water vapor (H2O) and nitrogen gas (N2).
The ability of the SCR catalyst to absorb ammonia may be a function of temperature. More specifically, a storage capacity of the SCR catalyst may decrease when a temperature of the SCR catalyst increases. Thus, excess amounts of ammonia may be stored in the SCR catalyst at low operating temperatures, which may result in a high NOx conversion efficiency. As SCR catalyst temperature increases, however, stored ammonia may be released from the SCR catalyst due to decreasing storage capacity. The released ammonia may affect NOx conversion efficiency because NOx sensors may detect the released ammonia as NOx. Furthermore, the released ammonia may be oxidized into NOx by a high temperature catalyzed PMF downstream, thus increasing NOx emissions.